Re: [gdal-dev] Re: JAVA API - Performance

Tue Nov 10 15:54:06 EST 2009

Caio Simone,

I just downloaded imageio-ext to check how it does that but it looks like I don't need to do that now, I can take you report instead. Thank you very much. I will take a look on array pinning for a start.

I translated the GDAL Proximity [1] code to Java and I timed both of then with the same input, a 1024x1024 byte image with just one pixel as feature at the center of the image.

It took 0.3 seconds in C++ and 1.5 seconds in Java!

I then translated the buffers to regular arrays and it went down a little bit, 1.3 seconds. 

It is still a big disadvantage. I believe that the buffer-to-buffer translation is the guilt time waster in that case.

[1] http://trac.osgeo.org/gdal/browser/trunk/gdal/alg/gdalproximity.cpp

My best regards,

Ivan

>  -------Original Message-------
>  From: Simone Giannecchini <simone.giannecchini at geo-solutions.it>
>  Subject: Re: [gdal-dev] Re: JAVA API - Performance
>  Sent: Nov 10 '09 12:36
>  
>  Ciao Even,
>  just wanted to add my 2 cents.
>  
>  As you know for the imageio-ext project we have been using the
>  GDAL-JNI bindings (actually a modified version of them) for a while in
>  order to allow Java users to leverage on GDAL using the ImageIO
>  framework which standard in Java.
>  This way we also enabled GeoTools and GeoServer to use GDAL as a datasource.
>  
>  In the past I have done quite some performance tests to add some
>  new/different methods to them and I can summarise our findings as
>  follows:
>  
>  - DirectByteBuffer vs regular arrays -
>  DBB is expensive to allocate but prevent the VM from performing copies
>  when having to move data to and from java and native code since they
>  live on the native space not on the java heap; On the other side the
>  regular arrays are fast to allocate but they are "usually" copied when
>  moved across from/to java and native code since the JVM cannot leave
>  the native code mess with the java heap space since the garbage
>  collector would not be very happy about that. I said "usually" since
>  there is a technique called array pinning that we can suggest the JVM
>  to use to avoid the copy of regular array; however this mechanism is
>  not guaranteed to be implemented and/or to work on each call (same
>  reason as above, GC is not happy about this technique).
>  
>  If you can pool the DBB  and/or use a few large DBB, where the cost of
>  the copy would overcome the cost of its creation then DBB are much
>  better than regular arrays. As an instance I noticed that using when
>  reading striped tiff files regular arrays where faster, but as the
>  tile size increases (and therefore the cost of a copy overcomes the
>  cost of a DBB creation) the DBB performs much better
>  
>  - DirectByteBuffer and the impact on some JVM -
>  Now in the past we decided to stick with DBB and give
>  GeoServer/GeoTools users the capability to retile data on the fly.
>  However lately, during the WMS performance shootout we noticed on some
>  linux machines JVm soldi crashed, not nice (means restarting the
>  GeoServer!!!).
>  We investigated a bit in depth and the problem was that somehow the
>  JVM was failing to allocate some internal images during the rendering
>  process and then dying with a NullPointerException (apparently the SUN
>  Java2D engineers did not use to check for out of memory errors in the
>  java native space). Well, what happens is that if you use too much of
>  the Java native space for your own objects, it is likely that the JVM
>  itself will start to malfunction (you can find articles on the web on
>  the memory model of a Java process, I don't think I am good enough to
>  explain it ) since it cannot allocate its own objects.
>  
>  In the end we decide to leave DBB and go back to regular arrays with
>  array pinning. This ensured us robustness and we did not see much
>  performance degradation (which means that array pinning in the end
>  works). This has been implemented by modifying the SWIG bindings for
>  GDAL in order to use a byte array instead of a DBB and then use
>  ByteArray utils to convert between different native type (short, int,
>  etc..).
>  
>  - Conclusion -
>  We might want to spend some time in the mid term to contribute some of
>  this work back (or probably provide funding), but anyway, it would be
>  great to have the capability to switch between DBB and regular arrays
>  since both have flaws.
>  However atm if I were asked I would say to go with regular arrays as
>  we do in the imageio-ext project.
>  
>  Ciao,
>  Simone.
>  -------------------------------------------------------
>  Ing. Simone Giannecchini
>  GeoSolutions S.A.S.
>  Founder - Software Engineer
>  Via Carignoni 51
>  55041  Camaiore (LU)
>  Italy
>  
>  phone: +39 0584983027
>  fax:      +39 0584983027
>  mob:    +39 333 8128928
>  
>  
>  http://www.geo-solutions.it
>  http://geo-solutions.blogspot.com/
>  http://simboss.blogspot.com/
>  http://www.linkedin.com/in/simonegiannecchini
>  
>  -------------------------------------------------------
>  
>  
>  
>  On Tue, Nov 10, 2009 at 12:00 PM, Even Rouault
>  <even.rouault at mines-paris.org> wrote:
>  > Selon Ivan <ivan.lucena at pmldnet.com>:
>  >
>  > Ivan,
>  >
>  > thanks for your testing (CC'ing the list as it is of general interest).
>  > Actually, I also read on some sites that using ByteBuffer object versus regular
>  > Java arrays is not always a win. Plus the fact that we must use a direct buffer
>  > that has an extra allocation cost according to the Javadoc. So ByteBuffer might
>  > be interesting if you just want to pass big arrays between native code, for
>  > example if you read an array from a dataset and then write it to another one
>  > without accessing it from the Java side. When you mention that accessing through
>  > the byte[] array was faster, did you get it with the array() method instead ?
>  > I'm wondering what the performance overhead of this call is.
>  >
>  > As ByteBuffer is not at all a requirement for the interface with the native
>  > code, it would be technically possible to add an alternative API that would use
>  > the regular Java array types.
>  >
>  > Would you mind opening an enhancement ticket about that ? Thanks
>  >
>  > Even
>  >
>  >> Even,
>  >>
>  >> I did some test with the GDAL Java API and some simple raster operations
>  >> like the GDAL Proximity algorthm and I noticed that the performance while
>  >> accessing pixels with <type>Buffer.get(i), <type>Buffer.put(i,value) is not
>  >> as good as if you copy then to (or from) a "regular" array, like float[],
>  >> double[], integer[] and byte[].
>  >>
>  >> The reason for that is obvious, get() and put() are funtion calls and
>  >> contains a lot of code for range check.
>  >>
>  >> If I understand it correctly, ByteBuffer is the ideal or maybe the only
>  >> way to get access to Buffers from C libraries thought a Java wrapper. But
>  >> do you it would be possible to incapsulate the buffer conversion at the
>  >> wrapper code so that users would be able to read and write direct to
>  >> regular Java arrays?
>  >>
>  >> Just a suggestion,
>  >>
>  >> Ivan
>  >>
>  >
>  >
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